1. Field of the Invention
The present invention relates generally to battery separator systems and methods for preparing the same and particularly to composite absorber/polyvinyl alcohol (PVA) barrier separator systems for use, for example in cells of high energy density secondary alkaline batteries.
2. Description of the Prior Art
Fabricating techniques for fabricating battery separators for alkaline battery cells have included the technique of thermally adhering, at limited sites, a thin polymer film separator to a synthetic, fibrous, heat sealable electrolyte absorbent sheet whereby a composite thin film separator/absorbent sheet is formed and maintained throughout handling and within a finished cell. For a discussion of this technique see U.S. Pat. No. 4,220,693 issued Sept. 2, 1980 to Ralph B. DiPalma and Anthony Loh, Jr. Such a composite structure appears to provide improved handling characteristics and dimensional stability of thin film separators as well as to eliminate a step in cell manufacture whereby installation of a separate electrolyte absorbent member is eliminated. However, this thermal adhering approach for forming a dual function separator seems to be an ineffective technique since spot thermal adhering two sheets together may cause spotty densification of the separator material, resulting in regions of the separator to be impermeable to both diffusion and hydrodynamic flow of electrolyte ions.
Another technique that has been employed which eliminates the need for providing a sandwich-type system, i.e., a barrier attached to an absorber in a battery cell, is to form a composite separator by laminating; for example, a sheet of lightly crosslinked PVA film is laminated to a top surface of a sheet of absorber material. Such a technique can provide an effective battery separator system; but, however, laminating techniques are usually sheet uniting means requiring subjecting the composite structure to relatively high levels of heat or adhesives and pressure. When pressure is used in conjunction with heat or adhesives, a new and different material composition results at the interface that differs substantially from either of the materials that are being united. Subsequently, laminating techniques inherently provide material between the layers which react differently to electrolyte flow than either of the materials that are united. It is reasonably obvious that both the surface and volume resistivity of the composite material formed in this manner are not enhanced by such a condition. Also, such an approach is probably a very expensive process since large quantities of crosslinked PVA film structures, suitable for laminating processes, are unavailable commercially.
The use of PVA films as barrier separators in a separator system is well known. In general, PVA material has been used as a barrier material in alkaline battery cells because suitably lightly crosslinked films have high mechanical strength and high hydrophilic properties. Also, suitably lightly crosslinked films can be fabricated readily from aqueous solutions despite the fact that noncrosslinked PVA will readily dissolve in water. This characteristic of PVA film provides a desirable attribute useful in battery separator systems.
But, however, most prior art methods for moderately crosslinking PVA to the extent required to form suitable barrier material for battery separator systems has required casting, onto base material such as a glass sheet, a film of noncrosslinked PVA from an aqueous solution. Then the PVA film which had been treated previously with a reagent that inhibits dissolution of the film, is contacted either by an acid catalyst or by an ionizing radiation technique in such a manner as to effect moderate insitu crosslinking of the PVA sheet. This moderately crosslinked PVA sheet is removed from the glass sheet base and then laminated or spot bonded to an absorber-type material to form the composite separator/absorber separator.
Various techniques and methods to achieve lightly crosslinked PVA film structures have been employed to improve the mechanical properties and chemical stability of PVA film for use as barrier material in alkaline battery cells.
U.S. Pat. No. 4,154,912 which issued May 15, 1979, to Philipp et al describes a two step method for forming an insitu self crosslinked PVA separator.
U.S. Pat. No. 4,218,280 which issued Aug. 19, 1980 also of Philipp et al describes an irradiation technique for crosslinking a PVA film which also had been cast on a sheet of glass.
U.S. Pat. No. 3,951,687 dated Apr. 20, 1976 of Takamura et al describes a PVA separator formed by coating both sides of a porous alkaline resistant nonwoven substrate with a mixture of an aqueous PVA solution and at least either of one selected from boric acids and metal oxides having low solubility to alkali solution and then drying the nonwoven fabric thus coated. Such procedure taught by Takamura et al could be used to mass produce PVA separators for sandwich-type use but by coating both sides of the substrate with the mixture appears to increase the volume resistivity of the separator material.
Another method for forming a PVA separator is described in U.S. Pat. No. 4,037,033 dated July 19, 1977 also of Takamura et al. There, a nonwoven fabric is pretreated by soaking it in a solution of a surfactant and then drying it. Then a mixture of PVA and an aqueous boric compound is coated over the treated nonwoven fabric covering all surfaces so that the separator does not have any large channels capable of being penetrated by dendrite growth. Such a procedure also could be used to mass produce PVA separators but it would consume considerable amounts of time and the separator is probably most useful in the sandwich-type separator systems.
A search for various other means of providing a PVA separator system which would eliminate the need for a separate absorber sheet and which could be mass produced readily was initiated. This search was successful and resulted in the present invention.